1. Field of the Invention
The present invention is directed to a method and system for registering images of a subject using magnetic resonance.
2. Description of the Prior Art
The use of magnetic resonance systems, with which images of different sectional planes of the body can be selectively made, is a routine application for diagnosing illnesses. Particularly when examining a body joint, it is often necessary to show the joint in its various angular positions. It is usually desirable that the desired tomogram plane, i.e. the image plane that xe2x80x9cis placed through the jointxe2x80x9d, always remains in the image in order to obtain reproducible images for different joint positions that can be compared to one another. There is, however, always the possibility that, due to the joint movement, the anatomical fixed points that originally were in the tomogram plane will migrate out of it, due to the anatomy of the joint and the range of motion associated therewith. Normally, the operator must manually readjust the measured slice in an image referred to as a xe2x80x9cscoutxe2x80x9d, which is measured after every attitudinal change of the subject, wherein three orthogonal slices are registered in rapid succession, after which the corrected slice of interest is redefined on the basis of corresponding operating aids.
Further, the publication xe2x80x9cActive MR Tracking Using an External Tracking Coil at 0.2 T for Scan Plane Registration During Kinematic Imaging of Moving Jointxe2x80x9d by X. Ma et al. (published in the conference volume of the ISMRM (International Society of Magnetic Resonance in Medicine), 1998, in Sydney, Australia) discloses re-adjustment of the tomogram plane of interest using a marker located at the body that defines this tomogram plane of interest. In this method, a coil provided with the element gadolinium xe2x80x94detectable with magnetic resonancexe2x80x94in a 1% solution was attached to the knee joint. The positional change of this coil due to movement of the knee joint was tracked with magnetic resonance, i.e. the magnetic resonance system provided for generating and registering diagnostic images is likewise employed here for tracking the movement of the coil. The procedure is such that, in alternation, the actual medical image is registered, the detection of the coil subsequently ensues, whereupon the actual image is registered again. An alternating mode is thus compulsory. This results in the imaging sequence necessarily being relatively slow, since the acquisition of the coil must repeatedly ensue in addition to the actual image registration. During this time, an image registration is not possible. In addition, a complicated control of the magnetic resonance registration system is required, since a switch must be continuously made between the different registration modes. In summary, even though a re-adjustment of the tomogram plane can be achieved with this method, it only allows low image registration speeds; and the required control and processing outlay is significant.
An object of the present invention is to provide a magnetic resonance imaging and system that enables a scan plane re-adjustment with a simultaneously high image registration rate in a simple way.
This object is inventively achieved in a method wherein the positions or one or more markings arranged at the subject and movable together therewith are detected with an acquisition system operated independently of and in parallel with the diagnostic image generating and registration mode of the magnetic resonance system, these positions serving as a criterion for the orientation of the subject relative to the diagnostic imaging system, and wherein, given a movement of the subject detected in this manner, the tomogram plane is re-adjusted dependent on the xe2x80x9cnewxe2x80x9d position of the marking or markings.
In the inventive method, the markings serving as points of reference with respect to which the orientation of the subject relative to the diagnostic imaging system is determined, and the markings are identified with the acquisition system that operates independently of the magnetic resonance system. Optical or electromagnetic acquisition systems are suitable for this purpose. This marking acquisition independently of the operation of the magnetic resonance system allows a continuous image generating and registration mode, since a changeover no longer has to be made in alternation between diagnostic image generation and mark acquisition, as is the case in known systems. At least one marking, and usually three markings, is/are located at the body in a fixed spatial attitudinal relationship relative to one another and move with the subject, The acquired position(s) of the marking(s) thus also move(s) as the subject moves, so that a change in the orientation thereof is acquired and, at the same time, the required change in the position of the tomogram plane can be determined therefrom, so that this plane can be re-adjusted dependent on the position(s) of the marking(s).
The position of one marking, or the positions of several markings fixed in position relative to one another, for example, in a small frame or a corresponding mount, are detected with the acquisition system in a first coordinate system. The position data of the marking(s) are transformed into a second coordinate system, in which the diagnostic imaging system generates the image data, using a transformation matrix that is computationally determined, with the follow-up adjustment of the tomogram plane ensuing in the second coordinate system. Using an adequately fast computer for determining the position data, the transformation matrix, the implementation of the transformation as well as the follow-up plane adjustments, a fast follow-up can be achieved without difficulty, which enables a continuous image registration. The transformation matrix is the link between the first, subject-related coordinate system and the second, system-related coordinate system. Via the transformation matrix, for example, the position and direction of the first coordinate system relative to the second coordinate system is known. When marking positions are acquired in the first coordinate system, these can be transformed without further difficulty into the second coordinate system with the assistance of the transformation matrix. Positional changes of the markings in the first coordinate system can be acquired relative to the second coordinate system, so that the plane re-adjustment can ensue dependent on the positional change.
For determining the desired tomogram plane, at least one further marking can be employed in accordance with the invention, the position thereof being acquired with the acquisition means and the scan plane proceeding therethrough. In this case, for example, the physician thus places this at least one further marking and thus defines a position that can be acquired and transformed by the acquisition system and through which the scan plane is then placed. The acquisition likewise ensues in the first coordinate system. In combination with the position data of the further marking, the position of the tomogram plane to be re-adjusted can be determined from a change in the position of the markings on the subject. Only one further marking can be employed, with respect to which a positionally fixed tomogram plane is defined, particularly a sagittal, dorsal or coronal scan plane. Thus, a stationary tomogram plane is placed through the single position prescribed by the further marking, with a plane selection from a number of possibilities ensuing as warranted. In order to have full freedom in terms of the plane position, three such further markings can be inventively provided through which the tomogram plane is placed. In this case, the position data of the three further markings, that define an entire plane in spatial position are acquired. A limitation to permanently prescribed, for example sagittal, transverse or coronal scan planes is no longer present in this case. Since, in the framework of the examination or treatment, it is often necessary for the physician that to define or modify the desired plane to be observed only during the actual examination, a manual pointer instrument, particularly in the form of a pen, can be employed, which the physician manually positions relative to the subject under examination, i.e., for example, the joint, and that is provided with (carries) the marking or the further markings. This instrument thus is used to position the further markings, at least until they are acquired by the acquisition system. Once the desired plane has been defined, it can be re-adjusted without further steps, so that the pointer instrument no longer has to remain in the designated position.
Alternatively to employing, further markings for determining the tomogram plane in terms of its position, the desired tomogram plane can also be manually defined, particularly using a keyboard or the like. In this case, for example using the computer keyboard or a track ball or the like, a specific plane is defined in the first coordinate system, this then being re-adjusted dependent on the acquired marking positions, as described above.
As mentioned, an optical acquisition system or an electromagnetic acquisition system can be employed as the acquisition system, with correspondingly fashioned markings. A stereoscopic camera is suitable as an optical acquisition system; reflective elements, for example in the form of small balls, that always reflect regardless of their position can be employed in this case as markings, and (if used) as further markings. In the electromagnetic embodiment, a transmission coil preferably operated with alternating voltage can be provided as a marking, and (if used) as further markings, and three reception coils aligned in different spatial directions and fixed in position relative to one another can be provided as an electromagnetic acquisition system. Such alternating current operation of the induction coils is unproblematical insofar as the fields thereby generated do not influence the fields employed in the framework of the diagnostic magnetic resonance examination. The positions of the marking can be continuously or discontinuously acquired.
In addition to the aforementioned method, the invention is directed to a magnetic resonance system for the implementation of the method, having an acquisition system operable independently of and in parallel with the diagnostic image generating and registration mode of the system for acquiring the position data for one or more markings attachable to the subject which are movable together therewith, the position data serving as a criterion for the orientation of the subject relative to the diagnostic imaging system and, thus an indicator for the movement of the subject. The position data are identified in a first coordinate system, having means for the transforming of the position data into a second coordinate system in which the image data are generated by the diagnostic system, and having means for automatic re-adjustment of a tomogram plane dependent on the identified position data.
Further, the inventive system also can include means for defining to tomogram plane, at least one further marking whose position can be acquired with the acquisition system in the first coordinate system and through which the scan plane proceeds being allocated thereto. This means for defining the tomogram plane can be a suitable computer that is capable of correspondingly defining and re-adjusting the tomogram plane. Only one further marking can be provided, whereby a positionally fixed, predetermined tomogram plane, particularly a sagittal, transverse or coronal tomogram plane, can be defined relative to this marking by the tomogram plane defining means. Alternatively, three further markings can be provided, the tomogram plane defined in terms of the data obtained thereby then being placed therethrough. The one or more markings can be produced using a manual pointer instrument, particularly in the form of a pen that can then be correspondingly positioned by the physician.
Alternatively, the means for defining the tomogram plane can be a device for the manual definition of the tomogram plane in the first coordinate system, particularly in the form of a keyboard or the like such as a track ball, joystick or some other suitable device. An optical acquisition system can be used, particularly in the form of a stereoscopic camera with allocated, reflective markings. Another alternative is an electromagnetic acquisition system having three reception coils fixed in position relative to one another and aligned in orthogonal spatial directions. A transmission coil is then utilized as the marking, or further marking. Regardless of the type of acquisition system which is used, a continuous or discontinuous acquisition is possible therewith.